THE CHEMICAL NATURE OF FIRE: A DETAILED ANALYSIS, PART-1

 Fire is dangerous. But sometimes, it is the fearful things that drive the chariots of modern civilizations. If handled with care, fire can create wonders, and build some of the most elegant things ever known to man. Used reluctantly? Then fire can burn down billions of dollars worth of property and life in a jiffy. 

A million years ago, our ancestors remained afraid of the dark. And it was the discovery of fire that helped to thaw their fear. How they discovered fire is a deeper mystery to date. But the early Homo-Sapiens and Neanderthals gradually learned the use of fire to cook meat, soften raw vegetables, and to scare away ferocious animals. As their lives became easy, they got more time to think and make sense of their surroundings. In due course, they forged weapons to attain dominion over all life forms, and evolve into today's modern man.

 A Glowing fire           source:Photo by form PxHere

But what is fire? Is it a state of matter? What do we actually see in the name of fire? These are interesting questions and to answer them we need to delve deeper into the true nature of fire. Thus, we need to analyze fire with the help of chemistry and a pint of philosophy.

Disclaimer: This is going to get rough for some readers. I need to illustrate the chemical nature of fire in three different parts. The link to the next part is given at the end.

                   If you don't possess a general knowledge about chemistry then voila! Skip those fussy symbols, equations and signs, and just read along. I think you won't be bored. So let's begin.

To understand the chemical nature of fire it is always advisable to brush up our high school knowledge of chemistry. Here are some key concepts:

Fire Triangle: For a successful fire, we need three things to work in harmony. Those are fuel(something that is to be burned/that can undergo a combustion reaction), air supply(typically oxygen/sometimes some oxidizing agent) and finally heat(to supply external energy/activation energy to start the chemical reaction). 

Heat and Activation Energy: To initialize a fire/combustion we need to supply external energy, in the form of heat. The molecules of the reactants(substances to be burned) are bonded to each other by chemical bonds. A combustion reaction cannot start if the molecular bonds are not broken. So we need heat/thermal energy to excite the molecules and break the chemical bonds. As a result the bond energy is released putting forth the first step towards a successful chemical reaction.

Combustible Material(Reactant): Combustible material/fuel/reactant is something that will actually undergo a chemical change. As the external energy is supplied the reactant species undergo a change in their chemical structure. Volatile substances vaporize and start to react with oxygen, producing heat, light and product species. During the combustion of a hydrocarbon substance like coal, oil or natural gas the products formed are typically  CO₂(carbon dioxide) , H₂O(water vapor), CO(carbon monoxide) and oxides of Nitrogen and Sulfur.

Combustion: Combustion is a rapid decomposition reaction that occurs between reactant species and  oxidizing agents/oxidant with liberation of energy. A reactant typically gets oxidized by the loss of electrons and the oxidant gets reduced by the gain of electrons. Whenever a substance is on fire it undergoes a chemical combustion. For example, every hydrocarbon combustion releases two products- CO₂ and H₂O.

           Few examples of combustion reaction are as follows(g stands for gaseous state of matter, s for solid and l for liquid):

• Combustion of methane: CH₄(g)+2O₂(g)→CO₂(g)+2H₂O(g)
• Combustion of ethane: 2C₂H₆(g)+7O₂(g)→4CO₂(g)+6H₂O(g)
• Combustion of methanol: 2CH₃OH(g)+3O₂(g)→2CO₂(g)+4H₂O(g)
• Combustion of octane: C₈H₁₈(g)+25O₂(g)→8CO₂(g)+9H₂O(g)
• Combustion of hydrogen:  2H₂(g)+O₂(g)→2H₂O(l)

But combustion doesn't necessarily mean the production of bright, warm flames. In this case, five different types of combustion deserves mention. They are as follows:

Fire for a good barbeque             source: Photo by form PxHere

1. Complete Combustion: Complete combustion happens in the presence of ample supply of O₂(oxygen) in air or pure O₂. Here the reactant species burn completely and form a limited number of product species. Like the combustion of a hydrocarbon in air. Complete combustion produces only CO₂(carbon dioxide) and H₂O(water) vapor. The combustion of H₂(hydrogen) in air produces H₂O. Further example would be the blue flame of a Bunsen burner when it undergoes complete combustion and releases maximum energy.
2. Incomplete Combustion:  Incomplete combustion happens in the absence of O₂ in air. Here the reactant species doesn't burn completely and produces residue of incompletely burnt fuel. Incomplete combustion of hydrocarbon produces CO(carbon monoxide), CO₂ and soot(particles of solid carbon).
3. Rapid Combustion/Explosive Combustion: Rapid combustion is the violent reaction of the reactant species that releases tremendous amount of heat, light and other forms of energy. Examples are thermobaric weapons that take up O₂ from air and generate a violent explosion.
4. Smoldering: Smoldering is a slow, flame-less form of combustion where O₂ reacts directly with the fuel surface. Solid materials can sustain smoldering, unlike oil and gas that vaporizes and burns off in flames. This is a form of incomplete combustion. Other examples are combustion of coal, wood charcoal, paper, steel wool, and cigarettes.
5. Spontaneous Combustion:  Spontaneous combustion is the combustion of reactant species due to the change in their internal chemistry. Certain substances catch fire if exposed to open air. This happens because those substances get immediately oxidized in air. Examples are freshly prepared charcoal, white phosphorus, compost or hay piles, etc.

Pyrolysis: Pyrolysis is the chemical decomposition of organic materials(carbon/hydrocarbon compounds) through the application of heat in absence of oxygen or in near absence of oxygen.

Oxidation-Reduction Reaction: Oxidation is the loss of electrons by the atoms of a reactant species and reduction is the gain of those electrons by the atoms of another reactant species. During combustion, the O₂ atoms donate an electron and the C(carbon) atoms gain an electron, thereby getting oxidized & reduced respectively.

Incandescence: Incandescence is the phenomenon of emission of electromagnetic radiation(mainly visible light) due to the application of heat to a material body. When heat is applied to a body it starts to glow in the visible spectrum. Soot or carbon particles in a fire emits a dull red and yellow light when heated. That gives the fire its combination of red-yellow-orange color. 

Flash Point: Flash point is defined as the minimum temperature at which a liquid fuel(petroleum products and petroleum derivatives) vaporizes enough to catch fire on exposure to a naked source of flame. Flash point of kerosene is 42-72℃.

 

Bombs Away!           source:Photo by form PxHere

This concludes our basic knowledge required to understand fire from atomic and molecular perspectives. Thus, we can now move on to the next part that deals with the idea of starting a fire and different types of fire. Here's the link to the next.